On the search for optimal damage precursors

A new approach to predict the service life of critical components via study of damage precursors is emerging and is the topic of this article. To date, most service life predictions are based on measurements of damage indicators and their growth toward criticality or failure, for example, fatigue crack length and material loss due to corrosion or wear. This makes lifetime estimates based on measurements of damage, for example, around half-life, or even at 80% life, difficult and inaccurate. To improve the accuracy and reliability of lifetime prediction, efforts are now underway to determine the state awareness of a critical component during service, based on property characterizations, in addition to the measurements of the direct damage indicators, such as crack length, acoustic emission, ultrasound signals, and eddy current measurements. These characterizations will include indirect damage indicators, that is, precursors and allied or affiliated damage indicators. For affiliated damage indicators, residual stress relaxation or development, phase changes, electrical property (resistivity, dielectric constant, permeability), and microstructural characterization must be considered. The selection of the optimal combination of direct and indirect damage indicators will be application specific. It is proposed to assess the efficacy of damage indicators on the basis of their D_i/D_fversus N_i/N_f, that is, damage ratio versus life fraction curves (referred to as damage indicator ratio curves), searching for indicators with damage indicator ratio curves that best meet the needs of the application.